The invention generally relates to digital television signal receivers and, more particularly, the invention relates to a phase detector for a vestigial side band (VSB) signal receiver.
To accurately extract data from a vestigial side band (VSB) signal as used, for example, in a high definition television (HDTV) transmission system the VSB signal must be demodulated by an oscillator that recovers the data bearing real part of the base band VSB signal for input to the quantizer (slicer). FIG. 1 depicts a block diagram of a conventional phase tracking loop 100 for a digital television receiver. This conventional loop is defined in the xe2x80x9cGuide To The Use Of The ATSC Digital Television Standardxe2x80x9d, Document A/54, Advanced Television Systems Committee, Apr. 12, 1995. The standard suggests using a phase tracking loop that operates only upon a real signal (e.g., the in-phase (I) signal). As such, to determine a phase error of the VSB signal, the quadrature phase (Q) signal is derived from the I signal. Because the I and Q components of a VSB signal are related by a filter function that is approximately a Hilbert transform, a filter 104 is used to derive a Q signal from the I signal. The filter is a finite impulse response (FIR) filter having fixed anti-symmetric coefficients and with every other coefficient equal to zero. The I signal is delayed in delay 102 for a period equal to half the length of the filter 104.
To compensate for the phase error, the I and Q signals are coupled to a complex multiplier 106 that is driven by an oscillator 112 (e.g., a numerically controlled oscillator (NCO)) that is, in turn, controlled by an error signal from a phase detector 108. The phase of the NCO output signal is modulated to correct the phase error and produce phase adjusted I and Q signals (Ixe2x80x2 and Qxe2x80x2). The phase detector 108 processes the Ixe2x80x2 signal and the derived Qxe2x80x2 signal to produce a phase error detection signal (e) that represents the phase error of the VSB signal. A loop filter 110 (a low pass filter) is used to filter the higher order components from the error signal e. In effect, the combination of the oscillator signal and the I and Q input signals within the complex multiplier 102 creates derotated Ixe2x80x2 and Qxe2x80x2 signals. The depicted circuit 100 is a closed loop phase tracking circuit that compensates for carrier phase error in the I and Q signals such that a relatively stable in-phase (Ixe2x80x2) signal is created that can be sampled within a demodulator to extract data from the VSB signal. One example of a phase tracking circuit that uses a derived Q component is described by Lee et al. in xe2x80x9cA Hardware Efficient Phase/Gain Tracking Loop For The Grand Alliance VSB HDTV Receiver,xe2x80x9d IEEE Trans. on Consumer Electronics, pp. 632-639, Vol. 42, No. 3, August 1996 and U.S. Pat. No. 5,706,057 issued Jan. 6, 1998.
FIG. 2 illustrates the real (I) and imaginary (Q) components of a VSB signal in the time domain. The figure represents the principal components of an isolated symbol, i.e., a symbol that is surrounded by many zero amplitude symbols and illustrates the Hilbert transform relation between I and Q components of a VSB signal. In a sequence of non-zero symbols, the waveforms of FIG. 2 are replicated along the time axis at the symbol period. The negative amplitude symbols have waveforms that are inverted from those of FIG. 2. As such, the imaginary part of a given symbol contains energy that is contributed by its two adjacent symbols. For example, in a sequence of three symbols, if the first symbol (xe2x80x9cbeforexe2x80x9d symbol) and the third symbol (xe2x80x9cafterxe2x80x9d symbol) have a different polarity, the imaginary component of the second symbol will have either a large positive or large negative value. In contrast, when the first symbol and third symbol have the same polarity, the imaginary component of the second symbol will have a small value. This inter symbol interference (ISI) causes a phase error in the detected phase of the VSB signal that would not be corrected by the phase tracking loop of FIG. 1. In essence, the phase error produced by the communication channel is modulated by the relative sign of the symbols in a symbol sequence without regard to phase error caused by the channel. Consequently, this component of the phase error becomes random noise with respect to the symbol decoding process and can cause errors in the phase tracking process.
Therefore, a need exists in the art for a phase detector that uses symbol sign information to improve the accuracy of the phase detection process.
The disadvantages heretofore associated with the prior art are overcome by a phase detector that uses symbol sign information to aid the phase error detection process to determine a phase error of a complex signal such as a vestigial side band (VSB) signal. The phase detector comprises a subtractor having one input coupled to the I signal (i.e., an unquantized digital value that is sometimes referred to as a quasi-analog signal) and a second input coupled to the data output of a slicer (quantizer) representing the magnitude of a sample point within a symbol (i.e., a digital value representing one of M levels of the VSB signal). The unquantized I signal is subtracted from the quantized I signal to produce a xcex94I signal. The I signal is also processed to determine the sign of the present symbol. A before-after symbol processor determines the sign values of a previous (before) symbol and a later (after) symbol and compares those signs to the sign of the present symbol. A change in signs from the before symbol to the after symbol will produce a substantial imaginary component in the phase error signal that can easily be tracked by the phase tracking loop. If the signs of the symbols (before-to-after) do not change, then the imaginary component of the phase error will be very small and nearly impossible to track and compensate. As such, the loop is disabled when the before-after processor detects such a symbol sign combination. As such the symbol sign information is used to xe2x80x9ccorrectxe2x80x9d or xe2x80x9cgatexe2x80x9d the phase error. This phase error signal is used in a digital television receiver to extract a data signal from a VSB signal for demodulation.